The invention relates to electromagnetic energy, and more particularly, to electromagnetic exposure of planar materials.
One drawback with conventional waveguides is that the microwave signal attenuates as it moves away from its source. This attenuation versus propagation distance increases when lossy planar materials are introduced into the waveguide. As a result, a material fed into the waveguide through a slot is heated more at one end of a segment (closer to a source) than at the other end (farther from a source). Prior art structures have not made use of the slot's orientation as a means for addressing this problem. In a traditional slotted waveguide, there is a field peak midway between two conducting surfaces. In the prior art, the slot is at this midway point. See, for example, U.S. Pat. Nos. 3,471,672, 3,765,425, and 5,169,571.
One way to address this drawback is disclosed in our co-pending and co-assigned application Ser. No. 08/848,244 now U.S. Pat. No. 5,958,275. Another way to address this drawback is disclosed in our co-pending and co-assigned application Ser. No. 09/350,991. In our two earlier applications, which are incorporated herein by reference, a path has a first conductive surface and a second conductive surface and a first end and a second end. A source is capable of generating an electromagnetic wave that propagates in a direction from the first end to the second end. The path has a slot that extends in a direction from the first end to the second end. The planar material is passed through the slot in a direction perpendicular to the propagation of the electromagnetic wave.
The structure disclosed in our two earlier applications is extremely useful for heating wider materials. In some applications, it may be advantageous to heat the material by passing the material in a direction parallel to the propagation of the electromagnetic wave. One possible way to heat a material by passing a material in a direction parallel to the propagation of the electromagnetic wave is disclosed in Metaxas et al, "Industrial Microwave Heating," Peregrinus on behalf of the Institution of Electrical Engineers, London, United Kingdom, 1983 (hereinafter, referred to as "Metaxas").
Referring now to FIG. 1, Metaxas discloses that a microwave power input 10 provides an electromagnetic wave (not shown) to a TE.sub.10 waveguide 30. The waveguide 30 has a miter bend 20 and rod supports 55. A conveyor belt 50 passes through a choke 42 along a path that is halfway between the top conductive surface 31 and the bottom conductive surface 32. FIG. 2 further illustrates that "[t]he conveyor belt is supported at intervals so that the mid-depth plane of the workload is coincident with the mid-points of the broad faces of the waveguide[.]" Id. at 114.
Miter bend 20 is usually referred to as a H-plane bend. In a H-plane bend, the long side a in FIG. 2 remains in the same plane. In an E-plane bend, the short side b in FIG. 2 remains in the same plane. In FIG. 1, the H-plane bend is oriented so that the electric field travels through the conveyor belt 50.
There are at least six drawbacks with the wave applicator disclosed in Metaxas's book. The first drawback is that the microwave signal attenuates as it moves away from the microwave power input 10. This attenuation versus propagation distance increases when lossy planar materials are introduced into the waveguide. As a result, a material fed into the waveguide 30 is heated more at the end of the waveguide closer to the input (end 33) than at the other end (end 34).
A second drawback is that the electric field is disrupted when the electric field travels through conveyor belt 50. In addition, there is better coupling if the electric field sees a narrow dimension, as opposed to a wide dimension, of conveyor belt 50. Metaxas fails to recognize that there is better coupling and the conveyor belt 50 is heated more uniformly if the electromagnetic wave travels across, as opposed to through, conveyor belt 50.
A third drawback is that a traveling wave is used to heat the planar material. Metaxas specifies on page 114 that "[i]n some cases where the workload has a very high loss factor, the traveling wave applicator is terminated in a short circuit because there is only negligible residual power. " Metaxas fails to recognize that it is possible to use a standing wave and continuously change the length or effective length of the waveguide or the frequency of the standing wave so as to even out the hot spots of the standing wave.
A fourth drawback is that the circular choke flange 42 is too wide at its widest point. Metaxas fails to recognize that a rectangular choke flange can limit the amount of energy that is lost through the opening.
A fifth drawback is that Metaxas does not disclose how to pass a planar material along more than one straight section of a serpentine waveguide. Metaxas specifies that "[a]t each end a miter bend (usually 90.degree. E-plane) permits connection to the generator and terminating load. The miter plates of the bends have holes with cutoff waveguide chokes to permit the belt and workload to enter and leave the applicator." Id. at 115. While Metaxas describes in the next section, meander (or serpentine) traveling wave applicators, Metaxas makes it clear that the material travels perpendicular to the long sections of the waveguide. Metaxas fails to recognize that it is possible to pass a material along (as opposed to across) multiple straight sections of a serpentine waveguide.
A sixth drawback is that in Metaxas it is not possible to heat just the edge of the planar material. In FIGS. 1 and 2, the entire conveyor belt 50 passes through the waveguide 30. In some applications, it is either not necessary or it is detrimental to heat the entire planar material. There is a need for a device that can heat just the edge of a planar material.